I graduated from a faculty of mathematics, so don't know if there are
other geeks who will find this amusing, but I came across this webpage:
 
  http://www.kleinbottle.com/whats_a_klein_bottle.htm
 
it describes the Klein bottle:
 
    Take a rectangle and join one pair of opposite sides -- you'll now
    have a cylinder.  Now join the other pair of sides with a half-twist.
    That last step isn't possible in our universe, sad to say.  A true
    Klein Bottle requires 4-dimensions because the surface has to pass
    through itself without a hole.
 
    It's closed and non-orientable, so a symbol on its surface can be
    slid around on it and reappear backwards at the same place.You
    can't do this trick on a sphere, doughnut, or pet ferret --
    they're orientable.
 
the base webpage is pretty amusing as well.
 
// ************************************************
// Selina, Birch, Dief, Sprite,       Sand and Bear
//               missing Storm
// http://www.cgl.uwaterloo.ca/~ssiu
[Posted in FML issue 3775]